Because I have a need for Manual Outbound NAT to force an outbound public IP AND implement failover for the rest of my machines inside the network, my question is:
How do I configure the Outbound NAT to use failover when a Interface is required for the rule?

NAT has nothing to do with where traffic goes, it only specifies how it's translated when it goes out the specified interface.

I understand this, but I'm telling you that with without an Outbound NAT rule assigned to the specific interface, outbound traffic does not work. If I disable (unplug) my primary Internet connection, the failover works properly, but I still have to manually change the Outbound NAT rule for "everything else" to use the secondary interface.